1. Field of the Invention
The invention relates to materials and methods for their use in defoaming.
2. Description of the Prior Art
The use of vegetable or mineral oil for defoaming aqueous media is well known. Silicone oils have proven to be more effective than these oils and, when used in only small amounts, are able to suppress the formation of foam on aqueous media or to eliminate foam that has already been formed. However, such silicone oils may cause difficulties during the further processing or use of the defoamed aqueous media due to the excellent spreading properties or the hydrophobic action of the silicone oil remaining in the system.
It is also well known that polyoxyalkylene monools or diols, or their ethers exhibit antifoaming or defoaming properties in aqeuous systems in which they are insoluble. Usually, the monools or diols of polyaddition products of ethylene oxide and propylene oxide have been used. Polyaddition products which contain oxyethylene units exclusively develop their defoaming action at or after passing their cloud point in aqueous solution.
In order to obtain products with improved defoaming properties, copolymers which comprise polyoxyalkylene and polysiloxane blocks, have also been synthesized. Especially preferred in this connection are block copolymers whose polyoxyalkylene block contain at least 80 weight percent of oxypropylene units. Such a preparation is described, for example, in DE-AS No. 24 43 853. With this preparation, the defoaming effectiveness can be increased even further by addition of microdispersed silica, which is produced, for example, by flame hydrolysis.
German Pat. No. 23 45 335 discloses a defoamer for aqueous solutions or dispersions consisting of 80 to 95 weight percent of a mineral, vegetable or animal oil, 1 to 7.5 weight percent of microdispersed silica or microdispersed aluminum oxide, and 1.0 to 10 weight percent of a methylpolysiloxanepolyoxyalkylene block copolymer, the methylpolysiloxane block of which constitutes 10 to 60 weight percent of the polymer and the polyoxyalkylene block of which contains 80 to 100 weight percent of oxypropylene units.
Most of the foam preventing or foam eliminating compounds known from the state of the art, are not resistant to attack by acidic or alkaline media. They are decomposed in the course of time, especially at elevated temperatures, and then lose their effectiveness.